Port timing questions

[redlinefever]

I have been looking for days so sorry for the repeat!! Anybody tell me how to figuire port timing and figuire blowdown??? If there is a link to it that would be a big help.

And again sorry for the repeat

 

[troutfisher]

To measure port timing, you're looking for how many degrees of crank rotation a port is open. To Measure an exhuast port I attach the degree wheel to the clutch with a magnet, and a piece of solder wrapped around the bar studs for a needle. Take the plug out and the muffler off.

Shine a light into the exhaust port and look in through the plug hole. rotate the crank until the piston moves down and just barely starts to open the exhaust port.

Set the wheel to zero

rotate the crank, and move the piston down past BDC and back up to where the port just closes. read the wheel, that is total exhaust duration.

Say you measure 160* total exhaust. Half of that is 80*, there is 180* per stroke so the exhaust opens at 100* ATDC. (160/2=80; 180-80=100)

 

[troutfisher]

You can pinch a piece of thin wire from the MIG in the top of the port to find zero also. That's how I measure transfers.

 

[redlinefever]

Thanks troutfisher!! I just rtying to get everything down before i start playing.
Got to work nights all weekend so i need a project to do at work might as well tinker with a saw.

 

[troutfisher]

Blowdown is how many degrees from when the exhaust opens until the transfers open. say you have 160* of exhaust and 120* of transfer.

160/2=80; 180-80=100 (exhaust opens at 100* atdc)

120/2=60; 180-60= 120 (transfers open at 120*atdc)

120-100= 20* of blowdown


check out this link....

http://www.2-stroke-porting.com/terminology.htm

 

[redlinefever]

That finally makes cents now !! Now i will have to try it out my self!!Rep comin your way!!!!!

 

[troutfisher]

Here's a printable degree wheel.......

 

[redlinefever]

Thanks again troutfisher!! Also have one more question when you machine a pop up how big of an area do you take out, just around the squish area or a little more ???

 

[troutfisher]

This is gonna sound a little goofy, but I sit down with some paper and scissors, and cut strips of paper that I can stick up into the cylinder and look at different shapes, depths, and diameters. Once I get the pop-up shape I want I can transfer that diameter to the top of the piston, and figure out how deep the combustion chamber is vs. how much I can cut off the piston, ect......

 

[HEAVY FUEL]

He's going to have to scratch off Dolmar and put Stihl on it.

 

[troutfisher]

Redlinefever, remember that this is just one method. Some guys like to find TDC with a piston stop, some guys don't use a degree wheel at all, they prefer a port map. They all work. Keep readin' and searchin' and find what works best for YOU.

 

[redlinefever]

well did the port map last night and probably going to degree it when i wake up !! It looks like maybe.375 ish wise piece machined for a little caution. Does the .015 still apply to it across the piston or just at the squish? Thanks again hoping to post pics of the piston after i open up the windows and polished them smooth!!

 

[redlinefever]

Alright got my # i think

Exh. total duration 180

intake total duration 141

blowdown 21

Transfer 116

Intake opens @ 69-70

exh 90

See you guys think these number are alright !!!!!

 

[blsnelling]

Alright got my # i think

Exh. total duration 180

intake total duration 141

blowdown 21

Transfer 116

Intake opens @ 69-70

exh 90

See you guys think these number are alright !!!!!

Something's not right here.

90° exhaust and 116° transfer openings are way high.

Also, if the exhaust opens at 90° and the transfers open at 116°, that's 26° of blowdown, not 21°.

 

[redlinefever]

alright went back do and took a look Brad. Checked exh first used a light in the spark plug hole and shined in rotated the crank until piston moved down and just a little light shine into the exh port got 95 degrees then shined light into cyl. and the top of the transfer port opend up (right at the top of the chamfer) 118 degrees so then i would have 23 degrees of blowdown. Right . Dang newbies!!!!!!! Thanks for the help brad i know it seem that you keep repeating your post over and over sorry

 

[troutfisher]

you can double check your own numbers. if the exhaust opens at 95*, total duration is 170*. that means there should be 190* with the exhaust closed. Same with the other ports. 170 + 190 = 360; and 95 + 85 = 180. Just proves to you that the wheel isn't slipping, needle moving, or anything else messing up your readings. If you can stay within a couple degrees you're doing good.

 

[redlinefever]

Thanks hey i also found out something else! Bought the new oem jug it has diffrent parrt numbers . The new cyl. has a little bigger piston in it any ideas.

 

[redlinefever]

checked my # again

intake @70
tranfers@118
exhaust @95
blowdown@23

what am i missing

 

[blsnelling]

alright went back do and took a look Brad. Checked exh first used a light in the spark plug hole and shined in rotated the crank until piston moved down and just a little light shine into the exh port got 95 degrees then shined light into cyl. and the top of the transfer port opend up (right at the top of the chamfer) 118 degrees so then i would have 23 degrees of blowdown. Right . Dang newbies!!!!!!! Thanks for the help brad i know it seem that you keep repeating your post over and over sorry

That sounds more like it but still quite high. Did you try going on around past BDC and seeing if you get the same reading on the way back up? That will confirm if you have TDC right.

 

[redlinefever]

Yeah brad same ## on the other side. so i dont know anyways c ya got to got to work again(just got off of a 12 hr shift now going to put up a couple grain bins!!! who needs sleep right)